The present invention relates generally to handling of bulk materials. More specifically, the present invention relates to a process for loading a large vessel with bulk material.
In recent years, greater emphasis has been placed on the use of very large cargo vessels for water-borne shipment of bulk materials. Vessels having cargo capacities on the order of 150,000 dead weight tons (DWT) are being used, and are being considered for use, in the transshipment of bulk materials. Many bulk materials are suitable for shipment by water including various ores, minerals, coal, grain, bauxite, phosphate rock, and the like.
One of the important economic considerations in transportation of any bulk material is the speed with which a particular volume of the bulk material can be loaded for transportation, transported and unloaded. This is especially important in the case of railroad transportation and water-borne transportation: in both rail transportation and water-borne transporatation demurrage fees must be paid when the train or ship is not loaded or unloaded within a specified period of time.
As the very large cargo vessels become more frequently used, existing material handling equipment at existing harbor facilities is strained to load or unload a large cargo vessel in the specified period of time. Accordingly, the need exists to develop a material handling system which is compatible with existing dockside facilities but which is also capable of quickly and efficiently loading very large cargo vessels.
A particular industry where the need for high capacity material handling systems has been demonstrated is the coal industry. Coal is enjoying a greater demand than in past years, for example, as a replacement for petroleum fuels in many power plant installations. Accordingly, large quantities of coal must be shipped from the coal fields to various power plant installations. Many of the large coal-producing field lie within a reasonable proximity to northeastern United States seaports. In addition, most coal fields enjoy existing railroad terminal connections directly with those same ports. These facts have caused considerable interest in the use of very large capacity cargo vessels to transport the coal. But, while most of those ports are provided with bulk loading facilities at dockside, most existing piers are incapable of efficiently handling the large capacity cargo vessels now being prepared for use.
The large capacity cargo vessels also experience another problem with the majority of east coast ports. That problem relates to the depth of the water available in those ports. Most each coast ports are themselves fairly shallow and have a water depth on the order of 40 feet. When a very large cargo capacity vessel is fully loaded, however, it may require a draft (water depth) of 50 feet or more. For example, a vessel having a cargo capacity of 150,000 DWT typically would require water depths of 52 to 54 feet. As a result, when a vessel having a cargo capacity of 150,000 DWT or more is employed in the shipment of bulk materials, that vessel cannot be fully loaded at the pier in most east coast ports.
Thus, the need exists for both methods and apparatus to facilitate the use of large cargo capacity shipping vessels while using the existing port facilities. Moreover the need continues to exist for minimizing capital costs while improving the cargo handling capacity of ports.
In the past, there have been some efforts to improve the bulk material handling capacity of ports. Among these efforts has been the use of self unloading vessels. Much of this effort has been centered in the Great Lakes area. Typically, a self unloading vessel has a slewable, luffable boom or has a shuttle boom. In either case, the boom itself includes a conveyor system. Generally one or more unloading conveyors run longitudinally beneath one or more cargo storage holds along the keel of the vessel. Each cargo storage hold is typically provided with gates to control the flow of material to the unloading conveyor(s). Generally the longitudinal conveyor communicates with an elevating conveyor which raises the bulk material to a deck level boom conveyor and discharges it onto the boom conveyor for off loading.
The boom of such unloading vessels is normally intended to off load onto a shore facility which is fairly low in comparison to the vessel itself. As a result, self unloading vessels cannot discharge material into the hatches of large capacity vessels which could be 40 feet or more above the deck of the self-unloading vessel when it is located alongside the large vessel. Another limiting aspect of the self unloading vessel is the positioning of the unloading conveyor(s) along the vessel keel. With such positioning of the longitudinal conveyor(s), the cargo hold must be provided with inclined sides at the bottom in order to allow gravity feed of the bulk material through gates to the unloading conveyor(s). Thus a considerable cargo volume is lost between the inclined sides and the hull resulting in wasted cargo capacity. Moreover, the center of gravity of the cargo is elevated since potential lower cargo space is wasted.
It has also been proposed to top off large cargo vessels from barges after partial loading at a conventional pier. But, this method appears to be limited in terms of vessel size. Many pier facilities use gravity feed systems. As a result the higher freeboard associated with large cargo vessels reduces the pitch of gravity system feed chutes thus interfering with gravity induced flows. And in any event, the vessel must later be topped off. Also, existing piers are not designed to accept the forces imposed by the docking of the large vessels.
One effort to enhance the removal of bulk material from the bottom of a cargo hold includes the use of a rotatable screw which is located at the bottom of the hold with its axis positioned transversely of the vessel and parallel to the bottom of the hold. The screw is rotated and simultaneously translated along the bottom of the hold. The direction of translation is perpendicular to the axis of the screw and in the direction of the longitudinal dimension of the vessel. This translating screw moves material from the bottom of the hold to a conveyor which is positioned to one side of the vessel. In addition, gates are required to contain the bulk material when the vessel is in transit. These existing translating screw installations exert very high thrust forces on the vessel that are unbalanced, suffer from complexity in the opening and closing of gates at the bottom of the hold and have a conveyor to which access for inspection and repair is difficult.